Methods and devices for gasification of solid and/or liquid fuels, including renewable fuels such as biomass are well known to those skilled in the art. See, for example, U.S. Pat. No. 4,699,632 to Babu et al. which teaches a process for gasification of cellulosic materials in a single gasification vessel in which the cellulosic materials are introduced directly into a single back-mixed fluidized bed of high heat capacity inert solids and U.S. Pat. No. 4,315,758 to Patel et al. which teaches the gasification of solid fuels to produce medium-Btu fuel gas or synthesis gas. Gasification of solid and/or liquid fuels is employed as a means for producing a gaseous fuel that will be substantially cleaner burning than direct combustion of the solid and/or liquid fuel. However, the synthesis gas produced by the gasification of solid and/or liquid fuels, in addition to being hot, also contains a significant amount of undesirable elements which must be removed prior to combustion of the synthesis
Methods and devices for the combustion of solid and/or liquid fuels such as pulverized coal, refused derived fuel, municipal solids waste and biomass are also well known to those skilled in the art. Such devices include stokers, fluid-bed combustors, pressurized fluid-bed combustors and pulverized coal combustors. See, for example, U.S. Pat. No. 5,205,227 to Khinkis et al. which teaches a process and apparatus for combustion of waste materials such as municipal solid waste, refuse derived fuel or other comparable solid waste in a stoker-type furnace. These methods and devices are typically employed in combination with industrial utility boilers for generating steam, which, in turn, may be used as process steam or in steam turbines for the generation of electricity. One problem with these methods and devices is the requirement for disposal of the significant amount of flyash and bottom ash generated. One partial solution to this problem is taught by U.S. Pat. No. 5,937,772 to Khinkis et al. in which flyash is recycled back to the combustion chamber for the recovery and further combustion of carbon present in the flyash. Another problem with these methods and devices is the frequent requirement for pre-drying of the fuel before introduction into the combustor, particularly with respect to renewable fuels such as biomass. Yet another problem with conventional systems is their intolerance of low heating value and variable heating value fuels which affects the ability of such systems to operate on a consistent basis, thereby compromising their capacity, efficiency, emissions and reliability.